The invention relates to motion tracking.
Motion tracking can use a variety of measurement modes, including inertial and acoustic measurement modes, to determine the location and orientation of a body.
Inertial motion tracking is based on measuring linear acceleration and angular velocity about a set of typically orthogonal axes. In one approach, multiple spinning gyroscopes generate forces proportional to the rates at which their spinning axes rotate in response to rotation of a tracked body to which the gyroscopes are attached. These forces are measured and used to estimate angular velocity of the body. Micro-machined vibrating elements and optical waveguide based devices may be used in place of gyroscopes.
Accelerometers generate signals proportional to forces which result from linear acceleration. In an inertial tracking system, the angular velocity and acceleration signals are integrated to determine linear velocity, linear displacement, and total angles of rotation.
As the signals generated by gyroscopic devices are noisy, the integration process results in accumulation of noise components, which is generally known as xe2x80x9cdriftxe2x80x9d. Miniaturized and low cost gyroscopic devices typically exhibit greater error. Drift rates can be as high as several degrees per second for a body at rest, and several degrees for every rotation of the body by 90 degrees. Errors in orientation estimates also affect location estimation as the estimated orientation of the body is used to transform acceleration measurements into the fixed reference frame of the environment prior to their integration. Inaccuracy in this transformation can result in gravity appearing as a bias to resulting horizontal acceleration measurements.
One way to correct drift is to use additional sensors, such as inclinometers and a compass to occasionally or continually correct the drift of the integrated inertial measurements. For instance, U.S. Pat. No. 5,645,077, issued to Eric M. Foxlin on Jul. 8, 1997, discloses such an approach. This patent in incorporated herein by reference.
Another approach to motion tracking uses acoustic waves to measure distance between one or more points on a body and fixed reference points in the environment. In one arrangement, termed an xe2x80x9coutside-inxe2x80x9d arrangement, a set of acoustic emitters at the fixed points on the body emit pulses that are received by a set of microphones at the fixed reference points in the environment. The time of flight from an emitter to a microphone is proportional to an estimate of the distance between the emitter and the microphone (i.e., the range). The range estimates from the emitters to the respective microphones are used to triangulate the location of the emitters. The locations of multiple emitters on the body are combined to estimate the orientation of the body.
Other measurement modes, such as optical tracking of light sources on a body, can also be used to track motion of the body.
In one aspect, in general, the invention is a method for tracking a motion of a body which includes obtaining two types of measurements associated with the motion of the body, one of the types comprising acoustic measurement, updating an estimate of either an orientation or a position of the body based on one of the two types of measurement, for example based on inertial measurement, and updating the estimate based on the other of the two types of measurements, for example based on acoustic ranging.
In another aspect, in general, the invention is a method for tracking the motion of a body including selecting one of a set of reference devices, transmitting a control signal to the selected reference device, for example by transmitting a wireless control signal, receiving a range measurement signal from the reference device, accepting a range measurement related to a distance to the selected reference device, and updating a location estimate or an orientation estimate of the body using the accepted range measurement. The method can further include determining a range measurement based on a time of flight of the range measurement signal.
Advantages of the invention include providing a 6-degree-of-freedom tracking capability that can function over an essentially unlimited space in which an expandable constellation of ultrasonic beacons is installed. Inertial measurements provide smooth and responsive sensing of motion while the ultrasonic measurements provide ongoing correction of errors, such as those caused by drift of the inertial tracking component of the system. Small and inexpensive inertial sensors, which often exhibit relatively large drift, can be used while still providing an overall system without unbounded drift. Small, lightweight inertial sensors are well suited for head mounted tracking for virtual or augmented reality display systems. By correcting drift using ultrasonic measurements, drift correction measurements which may be sensitive to external factors such as magnetic field variations, are not needed. The constellation of ultrasonic beacons can be easily expanded as each beacon functions independently and there is no need for wiring among the beacons. The tracking device only relies on use of a small number of ultrasonic beacons at any time, thereby allowing the space in which the tracking device operates to have irregular regions, such as multiple rooms in a building.
Another advantage of the invention is that by using an xe2x80x9cinside-outxe2x80x9d configuration, there is no latency in acoustic range measurements due to motion of the body after an acoustic wave is emitted.
Yet another advantage of the invention is that tracking continues using inertial measurements even when acoustic measurements cannot be made, for example, due to occlusion of the beacons. Drift in the inertial tracking is then corrected once acoustic measurements can once again be made.
In yet another advantage, the invention provides line-of-sight redundancy whereby one or more paths between emitters and sensors can be blocked while still allowing tracking of a body.
Other features and advantages of the invention will be apparent from the following description, and from the claims.